1. Field of the Invention
The present invention relates generally to glass compositions and, more particularly, to a high thermal expansion, high durability glass for hermetically sealing to high expansion materials such as copper, stainless steels, alloys of aluminum, and copper/beryllium alloys.
2. Description of the Prior Art
It is preferable to utilize glass as the electrically insulating medium for metallic components employed in electrical devices such as vacuum tubes, explosive detonators, pyrotechnic devices, accelerator switches, connectors, and the like. Glass is desirable for use in these applications because it is capable of forming a complex mechanical seal and is characterized in having a low gas permeability, a high chemical durability, a high mechanical strength and a high electrical resistivity. However, if high thermal expansion metals, such as aluminum or stainless steel, are employed in electrical devices, the glass utilized as the electrically insulating medium must also have a thermal expansion coefficient which closely matches the thermal expansion coefficient of the high thermal expansion metal. This close match is required to avoid mechanical stresses on cooling and permit a hermetic seal to be formed between the metal and the glass. As is well known in the art, forming a hermetic seal between the metal and the glass greatly reduces the possibility of moisture-induced denigration or failure of the metal.
Compositions formed from glass and ceramics for sealing to molybdenum are disclosed in U.S. Pat. No. 3,957,496 and for sealing to stainless steel are disclosed in U.S. Pat. No. 4,135,936. These compositions each require temperatures in excess of 900.degree. C. to form the seal and devitrify the glass. The high seal-forming temperatures of these compositions preclude their use in practice with metals, such as aluminum, since the seal-forming temperatures are greater than the melting points of these metals.
U.S. Pat. No. 4,202,700 to Wilder, Jr. discloses a glassy composition adaptable for hermetically sealing to aluminum-based alloys. The composition may either be employed as a glass or a glass-ceramic and includes from about 10 to about 60 mole percent Li.sub.2 O, Na.sub.2 O, or K.sub.2 O, from about 5 to about 40 mole percent BaO or CaO, from 0.1 to 10 mole percent Al.sub.2 O.sub.3 and from 40 to 70 mole percent P.sub.2 O.sub.5. Although this composition has a thermal expansion coefficient which closely matches the thermal expansion coefficient of stainless steel, its aqueous durability (dissolution rate) is relatively poor. This shortcoming precludes its use in electrical devices which require long operating lifetimes in humid environments.
It is apparent from what is presently known in the art that a need exists generally for a glass composition which may be hermetically sealed to high thermal expansion metals such as aluminum or stainless steel. In particular, a need exists for a glass composition for hermetically sealing to aluminum or stainless steel which has an aqueous dissolution rate at least an order of magnitude better than the dissolution rates of glasses presently known and utilized. In addition, the glass composition should have a thermal expansion coefficient which closely matches the thermal expansion coefficient of aluminum or stainless steel to permit the hermetic seal to be maintained as the aluminum or stainless steel expands and contracts, and a hermetic sealing temperature below the melting temperature of aluminum or aluminum alloys to permit the seal to be formed without melting either of the metals.